The present invention relates to hollow fiber membrane modules for use as what are termed “immersion modules” in filtration and/or dialysis methods, in particular whenever impairments due to fouling effects are expected on the basis of the use of liquids that are contaminated or lead to deposits, as well as to methods for producing such hollow fiber membrane modules.
Synthetic membranes have been used for separating materials for some years in industry, for example in sewage treatment or in biotechnology. In this case, a role is played, in particular, by the processing of aqueous systems, but also by the separation of gases or mixtures of organic liquids. In addition to membranes made principally from organic materials, for example polysulfones, there are also membranes that consist of inorganic materials such as, for example, aluminum oxide, carbon fibers and zirconium oxide, and which can withstand temperatures of up to 400° C.
By using pressure or underpressure, membrane filtration methods can be applied both continuously and discontinuously as ultrafiltration or, together with a concentration difference, as diafiltration. In the case of filtration volumes <1000 ml, filter cells are frequently equipped with membrane flat filters, whereas capillary or hollow fiber systems are used for larger volumes. The term capillary or tube membrane is used where the diameter of the tubular membrane >1 mm, and the term hollow fiber membrane when the diameter <1 mm, the diameter of a dialysis membrane typically being 0.2 to 0.5 mm.
Membranes for filtration or dialysis methods constitute thin film-like, either “dense” or porous separating layers. Depending on pore size, the porous separating layers are permeable only for specific molecule or particle sizes whereas, depending on the solubility and diffusivity of the materials to be separated in the substance of the separating layers, “dense” separating layers permit the materials to permeate faster or slower and therefore lead to separation. Membranes frequently have a foam-like support structure, with 60% to 80% hollow space, which supports the actual separating layer. Asymmetrically constructed membranes consist of a highly porous support structure in the case of which the size of the cavities inside the support structure decreases toward the side which supports the actual separating layer.
In order to process relatively large volumes of solution, tube-shaped membrane bundles made from hollow fibers or capillary membranes which permit a larger passage of solutions because of a large membrane surface, are introduced directly into the solution to be processed as what are termed immersion modules. In order to protect the membrane bundles against mechanical damage, which could be effected, for example, by forces arising from the liquid flow, the membrane bundles are frequently accommodated in a housing that lends the hollow fiber membranes sufficient protection and stability toward the outside. In this case, the housing has openings that are intended to permit the exchange of solutions between the housing interior, that is to say the hollow fiber membranes, and the medium in which the hollow fiber membrane module has been immersed.
In the design normally employed for a membrane module and which cannot be designated as an immersion module, the walls of the protecting housing have no openings, that is to say they are impermeable, and instead of this the housing has two connections, specifically a feed line and a withdrawal line through which the medium to be processed is fed to the outer surfaces of the hollow fibers and then led away again from the latter.
In the case of such normally used modules, the aim is as high a packing density as possible, which means that as many hollow fiber membranes as possible are accommodated in parallel in the housing interior, and the housing therefore has a high packing density. The term “packing density” refers in percentage terms to the ratio of the volume of all the hollow fiber membranes including their wall volume to the volume of the housing in which the hollow fibers are arranged. A high packing density therefore means a low volume for the hollow or free spaces inside the housing that are formed between the tubular membrane fibers. A natural upper limit to the packing density of <100% results from the fact that it is possible to arrange in parallel inside the defined volume of the housing only a number of tubular membranes which is such that they do not exhaust the prescribed volume. The limitation arises because when the tubular membranes touch interspaces are produced that are not tubular in shape and therefore leave over a residual cavity even given the most ideal arrangement. The packing density is, however, limited in addition by two further important factors. On the one hand, when potting the hollow fiber ends it is necessary to introduce sealing material between the hollow fibers and, on the other hand, the solutions being used are also to flow around the hollow fibers at the outer surface, so that, depending on the mode of operation of the module, it is possible either to bring a solution to be filtered into contact with the membranes, or, instead, to lead off filtrate. The hollow fiber bundles must be embedded at their ends in what is termed potting material so as to produce at each end in this way, just as in the case of shell-in-tube heat exchangers, a tube sheet denoted below as a “potting”. The result of this is the production, together with the housing into which the bundle is introduced, of two spaces that are separated by the membrane. The separated spaces thus produced can then respectively be provided with a feed line and a withdrawal line, in order to supply the feed to be treated to one space and to lead it therefrom as retentate, and to lead off the filtrate obtained from the feed from the other space.
The packing density of the normally used, conventional hollow fiber membrane modules is therefore approximately 25% to 30%.
Conventional hollow fiber membrane modules are designed chiefly for use in particle-free solutions, that is to say solutions or media that are not contaminated or do not tend to deposits. Such conventional, densely packed membrane modules with a perforated module housing are, however, frequently likewise used for technical processes in which particle-containing media, for example contaminated liquids in sewage treatment. Particularly in the case of such particle-containing media, what is termed fouling occurs in the course of the filtration process, that is to say in the course of time deposits that diminish ever more strongly the permeability of the membranes for the materials to be separated are increasingly formed on the membrane surfaces. This can go so far that the convective transport inside the hollow fiber membrane module, that is to say between the hollow filaments, is stopped completely, and that the transport performance of the entire module drops by orders of magnitude, since only a low percentage of the overall membrane surface area accommodated in the module still remains available for separating materials. If, for example, bundles of hollow fibers are used, it is possible, in particular, only for the hollow fibers arranged on the outer circumference of the bundle to participate in the convective transport of the exterior. In the case where the hollow fibers are arranged in the interior of the module, during the most exceptional case, it is only diffusion processes that still occur, but they are likewise greatly impaired because of the deposits.
It is normal to apply chemical or mechanical cleaning methods such as backflushing, mechanical vibration, ultrasound methods etc. in order to remove deposits from fouling processes. Apart from the fact that these cleaning methods are associated with a high outlay on energy, they always harbor the risk of mechanical damage to the hollow fiber membranes. If these customary measures can no longer be applied, the only measure remaining is a suitable incident flow onto the membrane surface through the feed solution.
In order to suppress fouling processes specifically in the case of applications in the revival pools of sewage-treatment plants, a filtration method (WO 99/29401; Zenon Environmental, Inc., Burlington, Ontario, Calif.) has been developed in which capillary membranes are introduced into the aeration tanks directly without a protective housing. In order to keep the fibers free from deposits, they are bathed with a uniform stream of air bubbles. However, these fibers have a partially substantially larger diameter than the fibers normally used, which have a diameter of less than 1 mm. They also have thick support structures. Moreover, these hollow fibers can be used only at such points in the aeration tank at which the mechanical forces induced by the flow are very small or can be kept very small. In this specific application, the spaces between the individual capillary membranes are up to several millimeters.
Also known in the art are commercially available hollow fiber systems in which several individual, tightly packed modules are accommodated in a housing, for example, with a packing density of 20 to 35%, in parallel interconnection. The result of this in principle is isolated strands between which there is sufficient space to ensure better incident flow or throughflow of the individual strands, and to minimize fouling processes. However, such systems require a technically very complicated housing whose production is correspondingly expensive.
In addition, use of what are termed spacers is known in the case of wound modules surrounding flat membranes, or electrodialysis stacks. The spacers are intended, for example, to ensure uniform spacings between the individual membranes and, on the other hand, simultaneously to distribute the flow of the solution into the respective feed-side or permeate-side compartments of the membrane module, in order thus to effect a flow over the entire membrane. The spacer materials used are reticulate structures with mesh sizes of different magnitude. However, these conventional spacer materials lead to an additional pressure drop in the liquid flow which can be equalized only by an additional expenditure of energy. Conventional modules with spacers therefore require a forced flow through their exterior.